The renal proximal tubule actively secretes many organic anions, including anionic metabolites and drugs. It is generally accepted that the basolateral membrane constitutes the active carrier-mediated step in transepithelial anion secretion. However, the physico-chemical nature of the carrier is obscure. Understanding the molecular characteristics of the membrane carrier is essential for predicting interactions between drugs and the kidney as well as understanding normal kidney function. We have identified and characterized a photoaffinity probe of this transport system, N-(4-azido-2-nitrophenyl)-2-amino ethylsulfonate (NAP-taurine). This compound is secreted by the kidney in the same manner as PAH and has a thirty fold greater affinity for the carrier. We have demonstrated we can use this probe to covalently label a maximum of four protein bands demonstrable by polyacrylamide gel electrophoresis of semi-pure basolateral membrane vesicles. We propose to purify our vesicle preparation and label the transport sites with NAP-taurine. A correlation between the labeled sites and the transport function will be evaluated by studying PAH transport in labeled vesicles and by radiation inactivation analysis. Finally, we will attempt to purify and reconstitute the components of the organic anion transport system. The NAP-taurine labeled protein may represent one of the specific carriers responsible for the translocation of organic anions across the basolateral membrane.